md/raid5: merge reshape_progress checking inside get_reshape_loc()

During code review, it's found that other than raid5_bitmap_sector(),
reshape_progress is always checked before get_reshape_loc(), while
raid5_bitmap_sector() should check as well to prevent holding the
lock 'conf->device_lock'. Hence merge that checking inside
get_reshape_loc().

Link: https://lore.kernel.org/linux-raid/20250227120452.808503-1-yukuai1@huaweicloud.com


Signed-off-by: Yu Kuai <yukuai3@huawei.com>